Heretofore, polyamides, typically, such as polyamide 6 (hereinafter abbreviated as PA6), polyamide 66 (hereinafter abbreviated as PA66) and the like, have been widely used as fibers for clothing and industrial materials and also as general-purpose engineering plastics because of their excellent characteristics and easiness in melt molding. On the other hand, some problems of poor heat resistance, dimensional stability insufficiency owing to water absorption and the like with those polyamides are pointed out. These days in particular, in electric/electronic fields that require reflow soldering heat resistance with advanced surface-mounting technology (SMT) and in automobile engine room parts and the like of which the requirement for heat resistance is increasing year by year, it has become difficult to use already-existing polyamides, and it has been desired to develop polyamides excellent in heat resistance, low water absorption, mechanical properties and physicochemical properties. Further, in coolant system parts applications such as radiator tanks, reservoir tanks or heater cores that use a long-life coolant, it has been desired to further improve the chemical resistance of polyamides.
For solving the above-mentioned problems with already-existing polyamides such as PA6, PA66 and the like, development of high-melting-point polyamides having a rigid cyclic structure in the main chain thereof has been promoted. For example, as polyamides having an aromatic ring structure, various semi-aromatic polyamides comprising, as the main component thereof, a polyamide of terephthalic acid and 1,6-hexanediamine (hereinafter abbreviated as PA6T) are proposed. PA6T has a melting point of around 370° C. or so which is higher than the decomposition temperature thereof, and therefore melt polymerization and melt molding with it may be difficult. Consequently, the polyamide is copolymerized with any other dicarboxylic acid component such as adipic acid, isophthalic acid or the like or an aliphatic polyamide such as PA6 or the like, and is used as a composition thereof whose melting point is lowered to a practicable temperature region of from 280 to 320° C. or so. In such a manner, copolymerization with a third component may be effective in lowering the melting point of the resultant polymer, which, however, is often accompanied by reduction in the crystallization rate and in the crystallinity of the polymer, and as a result, there occur various problems in that high-temperature rigidity (for example, the deflection temperature under load of polyamide at a temperature higher by about 20° C. than the glass transition temperature thereof is measured), chemical resistance, and various physical properties such as dimensional stability which water absorption affects worsen and, in addition, the productivity lowers owing to prolongation of the molding cycle. Further, relative to various physical properties such as dimensional stability which water absorption affects, the polymers are not still on a practical level though they are somewhat improved as compared with already-existing PA6 and PA66 owing to introduction of the aromatic group thereinto.
PTL 1 describes a polyamide composition that comprises a semi-aromatic polyamide comprising an aromatic dicarboxylic acid unit containing from 60 to 100 mol % of a terephthalic acid unit and a linear aliphatic alkylenediamine unit having from 6 to 18 carbon atoms, and contains a filler in an amount of from 0.5 to 200 parts by weight, relative to 100 parts by weight of the polyamide, and says that the polyamide composition is excellent in all of heat-resistant characteristics, mechanical characteristics, chemico-physical characteristics and molding characteristics.
On the other hand, as a polyamide having an alicyclic structure, PTL 2 discloses a copolyamide produced through copolymerization, which is a copolyamide comprising a polyamide component composed of 1,4-cyclohexanedicarboxylic acid and 1,9-nonanediamine and a polyamide component composed of terephthalic acid and 1,6-hexanediamine in a ratio by weight of from 25/75 to 85/15. However, the copolyamide has a low melting point and is therefore insufficient in point of the effect of improving the heat resistance thereof. A polyamide comprising a 1,4-cyclohexanedicarboxylic acid an aliphatic diamine having from 6 to 18 carbon atoms, which is described in PTL 3, is excellent in heat resistance, light resistance, toughness and low water absorption, but still has room for improvement in flowability.
Further, in a field of high-melting point polyamides, knowledge relating to the relationship between the moldability and the crystallinity of a high-melting-point polyamide is not sufficient.